The present invention relates to a process for the preparation of aldehydes and/or alcohols or, if required, amines by reacting olefins with carbon monoxide and hydrogen in the presence or absence of ammonia or a primary or secondary amine and in the presence of a catalyst which is homogeneously soluble in the reaction medium and contains at least one element selected from cobalt, rhodium or ruthenium in the presence or absence of a phosphorus-, arsenic-, antimony- or nitrogen-containing ligand at elevated temperatures and at superatmospheric pressure with the use of a jet loop reactor.
About 7 million metric tonnes of various products are produced annually worldwide by the hydroformylation of olefins. These are aldehydes, alcohols or amines. Aldehydes are produced essentially by the hydroformylation of olefins with cobalt carbonyl compounds or rhodium carbonyl or ruthenium carbonyl complexes which are homogeneously soluble in the reaction medium, as a rule rhodium carbonyl complexes whose reactivity and selectivity have been modified with a phosphorus-, arsenic-, antimony- or nitrogen-containing ligand.
Hydroformylation is understood as meaning the reaction of olefins with H2/CO mixtures, generally referred to as synthesis gas, to give aldehydes according to equation (1) 
in the presence of a catalyst from subgroup VIII of the Periodic Table of Elements. Furthermore, the hydroformylation under hydrogenating conditions, in which the aldehyde formed in the hydroformylation step is hydrogenated in situ in the hydroformylation reactor by the hydroformylation catalyst to give the corresponding alcohol, and the hydroformylation under aminating conditions, can be assigned to the area of hydroformylation reactions. Although it is also possible to use heterogeneous hydroformylation catalysts, the use of complexes of these elements which are homogeneously soluble in the hydroformylation medium has become established in the industrial application of the hydroformylation reaction. Usually, cobalt carbonyl, rhodium carbonyl, palladium carbonyl or ruthenium carbonyl compounds are used, these compounds being preferred with respect to their reactivity and chemoselectivity as a result of complexing with phosphorus-, arsenic-, antimony- or nitrogen-containing ligands.
The hydroformylation is usually carried out at elevated temperatures, and the preferred temperature ranges may vary depending on the type of hydroformylation catalyst used. Depending on the hydroformylation catalyst used and on the pressure range in which it is preferably employed in industry, a distinction is generally made between three types of hydroformylation, namely high-pressure hydroformylation, which is carried out at in general from 140 to 200xc2x0 C. and from 100 to 600 bar, medium-pressure hydroformylation, which is effected in general at from 120 to 180xc2x0 C. and from 30 to 100 bar, and low-pressure hydroformylation, in which temperatures at from 60 to 130xc2x0 C. and a pressure of 1 to 30 bar are generally used.
In general, different catalysts are preferably used for these different hydroformylation processes, namely, carbonyl compounds or hydrocarbonyl compounds, preferably of cobalt or rhodium, which have not been modified with additional organic ligands and form from readily obtainable precursor compounds under the hydroformylation conditions, in the high-pressure process under the reaction conditions used, cobalt carbonyl complexes modified with phosphorus-containing ligands, in particular phosphine ligands, in the medium-pressure hydroformylation and preferably rhodium carbonyl complexes having preferably phosphorus-containing ligands, in particular having phosphine or phosphite ligands, in the low-pressure formylation [sic].
The individual catalysts used in the various hydroformylation processes differ not only in their hydroformulation activity but also in their chemoselectivity, i.e. in their property of preferentially forming a specific hydroformylation product from among the isomeric hydroformylation products shown in equation (1) and in their property of having, in addition to the hydroformylation activity, also further catalytic activities which can be more or less desirable depending on the olefin to be hydroformylated and on the desired hydroformylation product.
Thus, many of the known hydroformylation catalysts additionally have hydrogenation activity which, depending on the reaction conditions used, is not inconsiderable, and possess said activity both for Cxe2x80x94C double bonds and Cxe2x80x94O double bonds of the carbonyl group. While the secondary reaction of the Cxe2x80x94O double bond is as a rule undesired since it leads to the formation of low-quality paraffins, the hydrogenation of the carbonyl groups of the aldehydes formed in the course of the hydroformylation to give the relevant alcohols may be entirely desirable since it dispenses with an additional hydrogenation stage which may be required. Such hydrogenation activity of the hydroformylation catalysts is also desired, for example, in the hydroformylation of olefins under aminating conditions, the imine or enamine formed from the resulting aldehyde or a primary or secondary amine present in the reaction medium being hydrogenated in situ to the desired amine.
Another secondary catalytic activity of some hydroformylation catalysts is the isomerization of double bonds, for example of olefins having internal double bonds, to xcex1-olefins, and vice versa.
Cobalt carbonyl complexes modified with phosphorus-containing ligands not only have, for example, hydroformylation activity but are additionally very effective as hydrogenation catalysts, and, depending on the CO/H2 ratio employed in the synthesis gas used for the hydroformylation, the aldehydes formed in the hydroformylation of the olefins with such cobalt catalysts are therefore completely or partly hydrogenated to the corresponding alcohols, so that alcohols or aldehyde/alcohol mixtures are formed, depending on the reaction conditions used (cf. B. Cornils in J. Falbe, New Syntheses with Carbon Monoxide, Springer Verlag, Berlin [1980] page 1-181)
The degree of these secondary activities of the hydroformylation catalysts can be influenced in the desired manner in some cases by establishing specific hydroformylation conditions in the reaction medium. However, often only small deviations from the process parameters optimized for the respective starting olefin and the desired hydroformylation product can lead to the formation of considerable amounts of undesired secondary products, and establishing virtually identical process parameters over the volume of the entire reaction liquid in the hydroformylation reactor may therefore be of considerable importance for the cost-efficiency of the process. Owing to their special process engineering aspects, this is true in particular for medium-pressure and low-pressure hydroformylation processes, the improvement of which is the subject of the present invention.
In the hydroformylation of, for example, an xcex1-olefin, straight-chain aldehydes, i.e. n-aldehydes, or branched aldehydes, i.e. isoaldehydes, may be formed, depending on which carbon atom of the olefinic double bond is involved in the addition reaction with the carbon monoxide (cf. Equation 1). For example, n-butylaldehyde and isobutylaldehyde are formed in the hydroformylation of propene. The commercial demand for the respective n- and iso-products obtained in the hydroformylation of specific olefins differs. Attempts are therefore made to produce these isomeric aldehydes in a specific n/iso ratio which corresponds to the demand for the individual isomers. The n/iso ratio can be influenced to a certain extent by establishing specific reaction parameters in the hydroformylation reactor.
In general, in the hydroformylation of olefins with the aid of ligand-modified homogeneous catalysts, it is advantageous to establish an optimum concentration of hydrogen and carbon monoxide dissolved in the liquid reaction medium, in particular the concentration of dissolved carbon monoxide, also abbreviated to [CO] below, is especially important. Particularly in the low-pressure hydroformylation of olefins with phosphine ligand-modified rhodium carbonyl complexes, even small deviations of the concentration of dissolved carbon monoxide of [sic] the optimum [CO] cause a deterioration in the result of the hydroformylation. The effect of different reaction parameters on the result for the low-pressure hydroformylation of olefins was scientifically investigated in great detail for the hydroformylation using rhodium carbonyl-triphenylphosphine (TPP) complexes, and this is therefore explained below taking these hydroformylation catalysts as an example, representing other ligand-modified catalysts which are used or can be used in the low-pressure hydroformylation.
The n/iso ratio of the aldehyde product produced in the hydroformylation with Rh/TPP catalysts is decisively influenced by the ratio of the carbon monoxide concentration to the triphenylphosphine concentration [CO]/[TPP] in the reaction liquid. Thus, thorough investigations by Cavalieri d""Oro et al (La Chimica e l""Industria 62, (1980) 572 have shown that there is a hyperbolic relationship between the [CO]/[TPP] ratio and the n/iso ratio, it being necessary to work in the region of the ascending branch-of the hyperbola to achieve a high n/iso ratio. If the [CO]/[TPP] ratio is reduced, the selectivity for the formation of the n-aldehyde, which is particularly desirable in many cases, increases. A reduction in this [CO]/[TPP] ratio can be achieved by decreasing the partial pressure Pco of the carbon monoxide in the gas phase of the hydroformylation reactor and/or increasing the TPP concentration. However, it must be borne in mind that the paraffin formation catalyzed as a secondary reaction by the Rh-TPP catalyst with hydrogenation activity increases when the CO partial pressure and hence also the concentration of dissolved CO in the reaction liquid decrease and the reaction rate is slowed down if the TPP concentration becomes too high.
The paraffin formation, the formation of high-boiling condensates of the aldehydes formed, i.e. high boilers, as well as the time-on-stream of the Rh-TPP catalyst are furthermore influenced by the reaction temperature. For an optimum hydroformylation procedure on an industrial scale, where large-volume reactors having a capacity of more than 100,000 metric tonnes/year are not unusual, it is therefore of decisive importance that no gradients with respect to the reaction temperature and the concentration of dissolved CO form over the volume of the reaction liquid present in the reactor, i.e. that identical operating conditions which are optimum for producing a desired n/iso ratio can be established over the total liquid volume. These facts are also explicitly referred to in DE-A 2810644, EP-A 254 180, U.S. Pat. No. 4,277,627, EP-A 188 246, EP-A 423 769 and WO 95/08525.
While the [CO]/[TPP] ratio is of particular importance for the hydroformylation result in the low-pressure hydroformylation with phosphine ligand-modified rhodium carbonyl complexes, in the medium-pressure hydroformylation with phosphine ligand-modified cobalt carbonyl complexes this is also true for the ratio of dissolved carbon monoxide and hydrogen, whereas in [sic] phosphite ligand-modified rhodium carbonyl complexes, as are likewise used in the low-pressure hydroformylation, may be sensitive to exceeding the optimum temperature range in the hydroformylation.
To keep the capital and operating costs for a hydroformylation plant as low as possible, but also for safety reasons, attempts are made to minimize the volumetric gas fraction eG at a specific temperature and a specific pressure, which is defined by equation (1)       e    G    =            gas      ⁢              xe2x80x83            ⁢      volume      ⁢              xe2x80x83            ⁢      in      ⁢              xe2x80x83            ⁢      the      ⁢              xe2x80x83            ⁢      liquid              total      ⁢              xe2x80x83            ⁢      volume      ⁢              xe2x80x83            ⁢      of      ⁢              xe2x80x83            ⁢      gas      ⁢              xe2x80x83            ⁢      and      ⁢              xe2x80x83            ⁢      liquid      ⁢              xe2x80x83            ⁢      in      ⁢              xe2x80x83            ⁢      the      ⁢              xe2x80x83            ⁢      two      ⁢              -            ⁢      phase      ⁢              xe2x80x83            ⁢      system      
and thus to maximize the space-time yield (STY). STY is understood as meaning the amount of olefins converted per unit time and unit volume, based on the total volume of the reactor. On the basis of the above, it is easily comprehensible that the STY is lower at a high total volumetric fraction eG in the reaction liquid, since the hydroformylation reaction takes place in the liquid phase and the excess gas volume in the reaction liquid occupies valuable reaction space uselessly. Since, in the industrial operation of the hydroformylation process, the chemical composition of the reaction liquid changes owing to the formation of hydroformylation products and byproducts, such as high boilers of different chemical composition, and the absorptivity of such reaction mixtures for the dispersed bubbles of the reaction gas is not known at every operating point, in extreme cases the hydroformylation reactor may overflow owing to an excessively high total volumetric fraction in the reaction liquid, this overflow being avoided by designing the hydroformylation reactor, in terms of engineering, with a larger volume than would be required for capacity reasons. Conversely, a reduction of the amount of dissolved carbon monoxide in the reaction liquid as a result of the formation of [CO] gradients in the volume of the reaction liquid owing to nonuniform gas mixing leads to lower conversions locally in the reactor and hence likewise to a reduction in the STY and to an increasse in the paraffin formation.
To prevent the formation of concentration gradients and temperature inhomogeneities in the reaction liquid, thorough mixing of the reaction liquid is required and it is for this reason that attempts are made to achieve ideal mixing of said liquid. For this purpose, EP-A 188 246, EP-A 423 769 and WO 95/08525 propose the use of stirrers or gasing stirrers or the use of the reaction gas streams passed into the reaction liquid for thoroughly mixing the reaction liquid. Gas distributors whose size, number and position in the reactor depend on the size of the reactor are used for distributing the reaction gas. The heat of reaction is removed with the aid of internal heat exchangers present in the reactor or of external heat exchangers.
The reactors proposed in EP-A 188 246, EP-A 769 and WO 95/08525 have the common disadvantages that the volumetric gas fraction in the reaction liquid can be changed virtually only by increasing or decreasing the gas streams passed through the reaction liquid. Specific control of eG by changing the stirrer speed is possible in principle but not very effective.
In view of the above information, it is self-evident that, in order to achieve optimum mixing of the reaction liquid in a large-volume hydroformylation reactor having a production capacity of, for example, 100,000 tonnes/year, stirrer constructions which are of very complicated design and are correspondingly expensive to precure must be used. It is for this reason that the alternative of using a plurality of smaller stirred kettles instead of a single large reactor is often employed in industry. This alternative likewise gives rise to high capital costs. A further disadvantage of the use of stirrers for the thorough mixing of the hydroformylation liquid is that the stirrer shaft has to be passed through the wall of the pressure reactor and this passage has to be provided with bearings and seals, which are exposed to considerable load and wear out in a relatively short time. Similarly, the stirrer blades or rotors are subjected to high mechanical stress. In order to replace these wearing parts, the reactor has to be switched off.
Since the hydroformylation reactor is highly exothermic and, as stated above, its selectivity is sensitive to concentration gradients and temperature inhomogeneities in the liquid reaction medium, thorough mixing of the reaction liquid must be ensured, otherwise there is the danger that the reactor will operate outside its stable range, for example due to local overheating, resulting in lower yields and selectivities.
When an increase or a reduction in the stirrer speed results in a change in the mixing of the reaction liquid, an increase or reduction in the stirrer speed has only a comparatively small effect on the volumetric gas fraction eG of the reaction liquid. Thus, the change in the stirrer speed is not a suitable measure for regulating the volumetric gas fraction eG in the reaction liquid.
As an alternative to stirred reactors, bubble columns are used in industry for carrying out the hydroformylation reaction. Here, the reaction gases are introduced at the lower end of the bubble column, via a gas distributor which ensures that the reaction gases are dispersed in the reaction liquid to increase the mass transfer surface area. Owing to their low density, the fine gas bubbles rise in the reaction liquid, with the result that the reaction liquid is thoroughly mixed. During the ascent, a part of the gases diffuses from the gas bubbles through the gas/liquid interface into the reaction liquid, in which they participate in dissolved form in the hydroformylation reaction. If the bubble column is operated at relatively low volumetric gas fraction eG through a correspondingly established feed of the reaction gases, [CO] concentration gradients and temperature inhomogeneities are formed over the length of the liquid column in the bubble column, with the described disadvantageous consequences for yield and selectivity. While a high volumetric gas fraction is required for ideal mixing of the reaction liquid, a large part of the available reaction space is occupied uselessly by the gas bubbles, causing the STY to decrease.
To solve the above problem, EP-A 254 180 proposes passing a part of the reaction gases into the reaction liquid via different feeds at different heights of the reactor in the form of a bubble column. As a result of this measure, a reduction in the [CO] gradient from, usually, 10% to 2% is reduced [sic].
DE-A 2810644 relates to the use of a flooded reactor, i.e. a reactor whose total volume is filled by the reaction liquid, for hydroformylation reactions in which the liquid and gaseous reactants [lacuna] fed into the lower part of the reactor and passed, in the interior of a tubular guide member which is present in the reactor and whose lower and upper end are each located a distance away from the reactor base and from the reactor cap, respectively, into the upper part of the reactor. There, the upward-directed stream of the reaction liquid is reversed so that it flows downward in the space between the wall of the guide member and the reactor wall, where, on meeting the reactor base or suitable deflection apparatuses, it is again deflected to become a stream flowing upward in the interior of the guide member. A part of the reaction mixture is removed continuously for product isolation at a withdrawal point in the lower part of the reactor, which withdrawal point is located so that virtually only reaction liquid from the downward-flowing stream is removed. The volumes of the reaction spaces with upward and downward flow are of roughly the same magnitude. This process can be carried out advantageously in the high-pressure hydroformylation of olefins using, as catalysts, cobalt carbonyls not modified with ligands, since the n/iso selectivity of these cobalt carbonyls is virtually not influenced by the concentration of the CO dissolved in the reaction liquid. The pressure used in the high-pressure hydroformylation with cobalt carbonylsxe2x80x94well above 100 barxe2x80x94is so high that virtually all the carbon monoxide fed to the reactor is present in solution in the reaction liquid, with the result that there is no reduction in the CO concentration of the reaction liquid. Merely to give a general idea, it may be mentioned here that the [CO] at 20 bar and 100xc2x0 C., i.e. under conditions typical in the rhodium low-pressure hydroformylation with phosphine ligands, is of the order of magnitude of about 200 g of CO/m3 of reaction liquid, whereas the [CO] at 280 bar and 100xc2x0 C. is in the region of a kilogram of CO/m3 of reaction liquid. Furthermore, the reaction rate in the high-pressure hydroformylation catalyzed with cobalt carbonyls is several times lower than in the low-pressure hydroformylation catalyzed with rhodium carbonyl-ligand complexes. Accordingly, it is also the object of the use of the reactor design described in DE-A 2810644 to increase the residence time of the reaction liquid by lengthening the distance to be covered by the reaction liquid in the reactor, in order to achieve a higher olefin conversion. The avoidance of gradient formation is not discussed in DE-A 2810644. Since, expressed visually, the apparatus of DE-A 2810644 is virtually a tube reactor which is inverted in the middle and, when operated at a reaction pressure as used in the low-pressure or medium-pressure hydroformylation, corresponds to a bubble column inverted in the middle, the use of this reactor design in the low-pressure or medium-pressure hydroformylation process is confronted with the same problems as those which occur with the use of bubble columns and as described above.
During the inventors"" preliminary work, it was found that the rate of the hydroformylation reaction is kinetically controlled in a wide parameter range, i.e. the rate of mass transfer from the gas phase to the liquid phase does not have a limiting effect on the reaction rate of the hydroformylation under the hydroformylation conditions-usually used. Consequently, the STY increases with decreasing volumetric gas fraction eG. However, the volumetric gas fraction eG cannot be reduced freely as desired since, at some point, the limit will be exceeded where the rate of the mass transfer from the gas to the liquid phase will have a limiting effect on the rate of the hydroformylation reaction. The boundary between kinetic control and mass transfer-influenced control of the reaction rate of the hydroformylation reaction is fluid. It is dependent, in a complex manner, on the method of mechanical introduction of power into the reaction liquid, on the specific phase boundary between gas and reaction liquid, expressed in m2/m3, and on the physical properties of the reaction system. An exact preliminary calculation has not been possible to date.
During the operation of an industrial reactor, the operator is faced with the dilemma that, on the one hand, establishing a high volumetric gas fraction eG in the reaction liquid for preventing gradient formation results in nonoptimum utilization of the reactor volume and hence a nonoptimum STY and, on the other hand, when the volumetric gas fraction eG is too low, there is the danger of gradient formation with adverse effects on the yield, the selectivity and the n/iso ratio, which, when a stirred reactor is used, can be counteracted only with economically unsatisfactory countermeasures. The same applies to the formation of temperature gradients over the volume of the reaction liquid it is an object of the present invention to provide a process for the low-pressure or medium-pressure hydroformylation of olefins which makes it possible to carry out the hydroformylation in a controlled manner at that volumetric gas fraction eG of the reaction liquid which is required for achieving an optimum STY and with that gas distribution and thorough mixing of the reaction liquid which, on the basis of a desired n/iso ratio, is required for achieving an optimum yield and selectivity, without this being associated with the economic disadvantages, described above, of the hydroformylation processes of the prior art.
We have found that this object id achieved by a process for the preparation of aldehydes and/or alcohols or, more preferably, possibly to amines by reacting olefins in the liquid phase with carbon monoxide and hydrogen, a part of these gases being dispersed in the form of gas bubbles in the reaction liquid and anther part being dissolved in the reaction liquid, in the presence or absence of a primary or secondary amine and in the presence of a cobalt carbonyl, rhodium carbonyl, palladium carbonyl or ruthenium carbonyl complex dissolved homogeneously in the reaction liquid and having a phosphorus-, arsenic-, antimony- or nitrogen-containing ligand, at elevated temperatures and at 1 to 100 bar, wherein the reaction is carried out in a vertically arranged, tubular reactor comprising a reactor body at least one circulation line, a part of the reaction liquid is fed continuously via the circulation line to at least one nozzle which is mounted in the upper part of the reactor body and is coordinated with a guide member, open at the top and bottom and bounded by parallel walls, in the interior of the reactor and with a baffle present below the lower opening of the guide member, and a downward-directed liquid stream containing dispersed gas bubbles is produced by means of this nozzle in this guide member and, after leaving the guide member, is deflected into a stream flowing upward in the space between the wall of the guide member and the wall of the reactor body and is sucked in to the guide member at the upper end of the guide member by the jet of the nozzle coordinated with the guide member.